Adding "const" to vpx_codec_iface_t is redundant

vpx_codec_iface_t is defined as follows:

  typedef const struct vpx_codec_iface vpx_codec_iface_t;

Since vpx_codec_iface_t is already a const struct, it is redundant to
add "const" to vpx_codec_iface_t.

Note: I think vpx_codec_iface_t should not have been defined as a const
struct, but it is too late to change that now.

Change-Id: Ifbd3f8a63c1d48e9169ff77fa0b505ea1e65519d